Many Gram negative and Gram positive pathogenic microorganisms produce toxins or hemolysins that can lyse eukaryotic cells and contribute to their pathogenicity. S. A. Mims (1982), The Pathogenesis of Infectious Disease (Academic Press). The most intensively studied exotoxin produced by Gram negative pathogens are members of the RTX family and are reviewed in A. Ludwig and W. Goebel (1991), Genetic determinants of cytolytic toxins from gram-negative bacteria; in J. E. Alouf and J. H. Freev (eds.), Sourcebook of bacterial proteins, pp. 117-146, Academic Press, London; and in R. A. Welch (1991), Mol. Micro. 5, 521-528, and V. Braun et al. (1991), Critical Reviews in Microbiology 18, 115-158. The HlyA hemolysin present in strains of uropathogenic E. Coli is the best studied of these. HlyA increases E. Coli virulence in a rodent model of peritonitis. R. A. Welch et al. (1981), Nature 294, 665-667. Although the precise mechanism by which this occurs is unclear, it is suggested that these toxins are involved in attenuating host phagocytic cell function. Welch et al., supra; S. Bhakdi et al. (1986), Infect. Immun. 52, 63-69; S. Bhakdi et al. (1990), J. Clin. Inv. 85, 1746-1753; S. J. Cavalleri et al. (1984), Infect. Immun. 37, 966-974; and B. Konig et al. (1986), Infect. Immun. 54, 886-892. Several facultative intracellular pathogens, including rickettsiae, shigellae, Trypanosoma cruz., and Listeria monocytogenes share a common trait, i.e., to allow escape from the phagocytic vesicle of professional phagocytic cells. J. W. Moulder (1991), Micro. Rev. 55, 143-190; J. Turco et al. (1991), Infect. Immun. 59, 1647-1655; B. B. Finlay et al. (1989), Micro. Rev. 53, 210-230; and N. W. Andrews et al. (1990), Cell. 61, 1277-1287. A defined role in virulence is difficult to assign to most toxins, in part because of the lack of convenient animal models for many organisms. One exception is the listeriolysin made by Listeria monocytogenes. The toxin dissolves the phagocytic membrane, allowing the bacteria to escape into the cytoplasm; hemolysin negative mutants are avirulent. J. L. Gaillard et al. (1987), Infect. Immun. 55, 1641-1646; S. Kathariou et al. (1987), J. Bact. 169, 1291-1297; and D. Portnoy et al. (1989), Infect. Immun. 57, 477-486. The infected cell remains intact, and the bacteria are protected from the immune defenses of the host. C. M. Hage-Chahine et al. (1992), Infect. Immun. 60, 1415-1421. Transfer of the gene-encoding listeriolysin to the non-pathogenic bacterium Bacillus subtilis allows it to also escape the phagosome, suggesting that listeriolysin is both necessary and sufficient for this event. J. Bielecki et al. (1990), Nature 345, 175-176.